Superabsorbent polymers are water insoluble, hydrogel-forming polymers capable of absorbing large quantities of aqueous fluids including synthetic urine, brines, and biological fluids such as urine, sweat, and blood, while retaining the absorbed fluids under pressure. Hydrogel-forming super-absorbent polymers are useful as absorbents for water and aqueous body fluids when the polymers are incorporated in absorbent articles, such as disposable diapers, adult incontinence pads, sanitary napkins, and bandages. Many of the existing superabsorbents are formed from unsaturated carboxylic acid monomers including acrylic acid, methacrylic acid, alkylacrylates, and acrylamides which are rendered water insoluble by crosslinking.
The degree of crosslinking affects the absorbent capacity and gel strength of a superabsorbent. Absorbency is a measure of the mount of fluid which a given mount of superabsorbent polymer will absorb. Gel strength indicates the tendency of the hydrogel once formed to deform under an applied stress. Polymers exhibiting inadequate gel strength will form a hydrogel which deforms and fills the void space in an absorbent article, inhibiting absorbent capacity and fluid distribution throughout the article. Polymers having low absorbent capacity are incapable of absorbing a sufficient amount of the fluid encountered in use of a diaper or other absorbent article. A polymer having a high gel strength generally possesses a low absorption capacity, and a polymer having a high absorption capacity typically possesses a low absorption rate because of gel blocking phenomenon or low gel swelling pressure.
Another characteristic that a superabsorbent polymer must possess is an acceptably low level of extractable, water soluble polymer remaining within the superabsorbent. The extractable polymer can leach out of a hydrogel when fluids contact the superabsorbent. So called extractable polymers contribute to a "wet feel" of the article, and slow fluid transport in the article resulting in leakage of fluid from the absorbent article.
Commercially available superabsorbents generally possess sufficient capacity, and absorption under pressure but do not have adequate gel strength, swell rate (i.e. absorption speed) and permeability under pressure for the absorbent articles of reduced size and thickness that are now being produced. As fluff fiber in absorbent products is replaced with greater amounts of superabsorbent polymer, the polymer has to perform the functions of the fluff fiber. The polymer must quickly absorb fluid and transport it throughout an absorbent article without releasing the stored fluid from the swelled hydrogel on exertion of pressure. Accordingly, the swollen gel particles cannot impede absorption of additional fluid by forming a barrier, but must maintain their liquid permeability.
In order to improve the absorption speed of superabsorbent polymers, blowing agents have been incorporated into superabsorbents as described in U.S. Pat. Nos. 5,118,719 and 5,145,713. As the blowing agent is dispersed throughout the monomer solution during polymerization, it releases carbon dioxide. The porosity of the resultant superabsorbent polymer provides more surface area within the polymer particles, increasing the rate at which fluid is absorbed by the polymer.
The absorption under pressure of a superabsorbent has been improved by crosslinking the molecular chains at the surface of the polymer. Surface crosslinkage also improved the gel strength of the polymer and reduces the amount of extractables at the polymer surface. Although capacity is reduced at the polymer surface, the core of the polymer, which has lower crosslink density, retains its absorbance capacity. Normally, surface crosslinking creates a random, fairly uniform shell of higher cross-link density. Surface crosslinkage, however, frequently reduces the absorption speed of the polymer. While these materials possess adequate absorption under pressure, they absorb significantly slower than the fluff fiber they are replacing in thinner personal care articles.
The polymers which have been crosslinked at their surface (herein referred to as core polymers) are not porous materials like those disclosed in U.S. Pat. Nos. 5,118,719 and 5,145,713. Accordingly, these superabsorbents generally exhibit slow rates of absorption. U.S. Pat. Nos. 4,666,983 and 5,140,076 disclose absorbent polymers formed from reacting an absorbent resin powder having a carboxyl group with a crosslinking agent having at least two functional groups per molecule to crosslink the surface of the polymer. German Patent No. 4,020,780 describes surface crosslinked superabsorbent particles formed by coating monomers having acid groups with an alkylene carbonate. U.S. Pat. No. 5,229,466 discloses surface crosslinking by treating a water swellable carboxyl group containing polymer with a solution of an N-(hydroxyalkyl)-beta-(meth)-alanine ester or a polycondensation product thereof. A superabsorbent polymer made from acrylic acid, a water soluble polysaccharide and a crosslinking monomer having at least two polymerizable ethylenically unsaturated double bonds per molecule is described in U.S. Pat. No. 5,145,906. Water absorbent resins surface treated with a polyquatemary amine are disclosed in U.S. Pat. No. 4,824,901.
Swollen gel permeabilities of commercial superabsorbent materials as measured by the test described herein (at 60 minutes) are often much less than 5 g/g, or less even for so called advanced materials. There is a need for superabsorbent materials having higher swollen gel permeability characteristics. Materials exhibiting higher gel permeability characteristics should and do exhibit properties allowing more efficient use of superabsorbent in articles by making use of a greater portion of the superabsorbent in the superabsorbent containing personal hygiene article. As an example, bodily fluids, such as urine in a baby diaper tend to be deposited in the crotch area of the diaper. One of the most difficult and limiting factors of diaper design is to have the diaper transport the urine away from the crotch, against the force of gravity, into the unused front and rear portions of the diaper. The driving force comes from the wicking and suction of other portions of the diaper. Most tests described in the literature measure swollen hydrogel permeability by passing an aqueous fluid through a confined, swollen hydrogel with gravity or a hydrostatic head as the driving force, for instance in U.S. Pat. No. 5,147,343. The test described herein more accurately requires the fluid to move against gravity through a compressed swollen matrix, with the driving force being the wicking ability and suction power of the hydrogel itself. The superabsorbent polymers described and claimed herein exhibit improved permeability, resulting in less leakage from personal care articles made therefrom, and greater use of the total of the superabsorbent in the article. There is accordingly a need for a method of producing a water absorbent resin which exhibits high absorbency under pressure, high absorption speed, high permeability under pressure, and high gel strength.